1. Field of the Invention
The present invention relates to wafer metrology, and more particularly to obtaining overlay measurements for a semiconductor wafer using periodic gratings formed on the wafer.
2. Related Art
Semiconductor devices/circuits are formed on semiconductor wafers by depositing and patterning layers of materials. In general, the features of the devices/circuits are formed onto the layers of deposited materials using a patterning process.
In a typical patterning process, the features of the devices/circuits are laid out, one layer at a time, on a series of photomasks (masks). The layout of the features of the devices/circuits on the masks are transferred, one mask at a time, onto the deposited layers of materials. Misalignment of these masks, which is generally referred to as xe2x80x9coverlay errorxe2x80x9d, can adversely affect the performance of the devices/circuits.
To reduce overlay error, alignment marks, such as box-in-box or frame-in-frame overlay marks, are typically patterned onto the wafer and on layers deposited onto the wafer. At present, optical imaging systems are widely used to detect these alignment marks. However, a conventional optical imaging system typically has an accuracy of only about 5 to 10 nm. The continual shrinkage in the feature sizes of devices/circuits will likely require greater accuracy.
In an exemplary embodiment, overlay measurements for a semiconductor wafer are obtained by forming a periodic grating on the wafer having a first set of gratings and a second set of gratings. The first and second sets of gratings are formed on the wafer using a first mask and a second mask, respectively. The first and second sets of gratings are intended to be formed on the wafer with an intended asymmetrical alignment. A diffraction signal of the first and second sets of gratings is measured after the first and second sets of gratings are formed on the wafer. The misalignment between the first and second sets of gratings formed on the wafer is determined based on the measured diffraction signal.